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Supension Truth #4: Once Upon a Bumphttp://www.thetruthaboutcars.com/2012/10/supension-truth-4-once-upon-a-bump/
http://www.thetruthaboutcars.com/2012/10/supension-truth-4-once-upon-a-bump/#commentsThu, 18 Oct 2012 15:12:18 +0000http://www.thetruthaboutcars.com/?p=464036A comment several articles ago on quantifying ride evaluation (the ‘ass-o-meter’ as one commenter put it) lodged in my brain. It reminded me that I never expanded on the additional data acquired during this magazine test three summers ago. I’ll do that here, add a few words on the timed track testing and then give […]

A comment several articles ago on quantifying ride evaluation (the ‘ass-o-meter’ as one commenter put it) lodged in my brain. It reminded me that I never expanded on the additional data acquired during this magazine test three summers ago. I’ll do that here, add a few words on the timed track testing and then give you hungry readers some enticing graphs, to balance out my verbose ramblings (wink wink!).

In June 2009, Grassroots Motorsports Magazine, based in Ormond Beach, FL, contacted me about providing an FCM coilover package and also coming out for the tire/suspension evaluation day using staff writer’s Tom Heath’s 1994 R-Package Project Miata on their regular test course, a go-kart track in nearby Ocala, GA. The overall timed results and related driving commentary are detailed in that article, telling part of the story. You can download it from or read it on our website (PDF) (link: http://fatcatmotorsports.com/GRMtest_FCM_miata_suspension.pdf ) .

Space constraints meant that I didn’t get to share all of the data gleaned from that day. My intention with this post is to provide more of that data (vertical acceleration and shock dynos) to illustrate some of the key elements of suspension tuning. For these tests, I utilized a speed bump in their parking lot, storing data on my laptop via our Signal Quest +/- 5g 3-axis box.

The timing of building/delivering our suspension was quite tight as they had a narrow window to do the testing, plus I’d be traveling right after the 4th of July weekend. Not much of a vacation that year! I finished valving/assembly the morning before my flight and checked it on the plane with me. Luckily, TSA let it through (leaving a ‘we’ve inspected this package’ note).

Having a closed circuit, a test driver who knows his car and and two full days was a dream come true. Tom’s Miata had a bit of oomph, delivering about 240 whp through its turbo-charged 1.8L engine, but it was still riding on the stock R-package Bilstein shocks. Bilstein had provided a brand new PSS9 adjustable setup, while I brought along an FCM Elite 3140 coilover suspension. I chose spring rates that would provide similar roll stiffness to the PSS9 setup, while also taking into account variables like the combined weight of the vehicle and its occupant. We used 425/300 lb/in spring front/rear, compared to the 342/342 on the PSS9 and ~190/110 of the stock R-package springs, which also used strongly rebound-damped Bilsteins. The R-package cornered on the ~240 lb/in bump stops very actively and sat very low from the factory, constantly pulling into the rubber bump stops like a race coilover from the factory.

Two items not mentioned in the GRM article were the time constraint and the weather conditions. We would have two days to perform all tests, including a few different tire widths/compounds on the stock suspension. Then, we’d leave the 225 NT01/15×9 wheel combo on for evaluating the three suspensions: R-package, PSS9 and FCM. For the second constraint, we had showers to contend with throughout both days. Rain would frequently douse the track, requiring us to wait until clouds cleared so the sun could dry the asphault sufficiently to test again … before the next shower.

The initial tire tests plus stock suspension evaluation took most of the first day. As the second day started, the Bilstein PSS9 went on and got 4 sessions of 5 laps each. It was immediately apparent the rear spring rate bias was too strong (or too soft in the front). Each session we made a change, e.g. cranked up the front damping (went to setting number 5 on the 9 way adjustable shocks), softened the rear and even removed the rear sway bar to tame the tail. It was late in the day when we called the PSS9 tests a wrap. I had to rapidly swap in the FCM suspension and pray the rain stayed away long enough to get some good runs. With clouds looming yet again, I set the heights to 11.75/12.0”, a bit lower than the PSS9 up front but same in the rear. No corner-balancing or alignment was done prior to any tests. We left the dampers at the nominal ‘best grip/ride’ setting of 2.5/3.0 turns F/R (out of 6) from full stiff and sent Tom out.

He only managed 1 test session of 5 laps with our FCM setup before the sprinkles started. It was already 4:30 so no chance for additional testing and no time to play with damper values which I’d really wanted to do. C’est la vie! Tom still managed to average a better time than either of the other two suspensions! But what’s anecdotal evidence without numbers to back it up?

Now, to the backstory of speed bump testing. Here are the R-package accelerometer graphs, followed by shock dyno plots, then the PSS9 and FCM. In each run, Tom drove 10mph over the bump, which had a trapezoidal shape and sharp vertical edges. While you’ll notice the accelerometer didn’t read exactly 1.0g, the baseline was consistent for each test.

The softer springs on the factory R-package helped mute the impact of the speed bump but the chassis was jacking up and down even before the speed bump was encountered (oscillations visible in the data from time 80-82s, then 84-86s). The upward acceleration was mild (~0.3-0.4g), but followed by a stronger downward pull (0.6-0.7g) on the descending edge. Again, note the oscillations and ‘ringing’ in the data which is due to literally bouncing off the bump stops from the excessive rebound damping. The overall time scale on this graph is DOUBLE that for the PSS9 and the FCM setup, indicating how long it takes this setup to stabilize after a disturbance. Peak to peak vertical g values are between 1.4 to 0.1g, over 1.3g of vertical acceleration. The result is an extremely harsh and jittery feel.

As we see here, there is much less rebound with the PSS9. The sharper vertical acceleration is likely due to proximity to the front bump stops. With the PSS9 setup at the recommended Bilstein height (approx 12” front and rear, which I felt was too low for the shock body length and their longer bump stops, 40mm vs. our 16mm), there is less jitter but higher vertical acceleration values. Some ringing still occurs after the speed bump (for about a second), showing the system is underdamped. There may be some influence from the rear frequency being 25% higher than front.

Peak to peak vertical acceleration is between 1.45 to 0g on the first part of the bump and 1.5 to 0.38g on the second half, so still between 1.1-1.4g.

Two features jumped out at me: First, the front and rear suspension both stayed closer to 0.2 g in rebound stroke while the PSS9 went below 0g on the front The PSS9 rear was better-behaved than the front and softer than the FCM rear. Second, the FCM setup had essentially zero ringing (before or) after the bump compared to the PSS9 and certainly the hyper-packing R-package. With 425/300 spring rates, the rear was about 5% higher frequency than the front. Peak vertical g’s at this height are about 1.5-0.2 and 1.4-0.2 or between 1.2-1.3g.

From the data, I felt the car with FCM suspension was too low in the front so I added 1/4” of ride height, to 12/12 front/rear. Tom settled the suspension and repeated the speed bump test:

This was truly sweet, and a great illustration of how important ride height can be to your ride comfort! Essentially 0.5 g of acceleration both upward and downward, with a small bias in the rebound direction on the rear, and perhaps a bit too much high-speed bump. I’d call this a neutral-jacking setup that promotes smooth settling, grip and aids confidence at the limit. A speed bump is a special case but it can show how a suspension will respond to sudden inputs or mid-corner bumps that are apt to throw a vehicle unnervingly off-line.

The whole experience with GRM was really gratifying, if stressful. I really enjoyed the camaraderie around cars and many areas of life. Amusingly, the picture of me for their article was from behind, head buried in the wheel well. An accurate depiction of where I spent most of my time! Now, fast-forward three years and I can see some revised damping, even stiffer springs and other changes to net more speed and perhaps comfort as well.

]]>http://www.thetruthaboutcars.com/2012/10/supension-truth-4-once-upon-a-bump/feed/3Suspension Truth #2: Sport Suspensions – The Illusion of Performancehttp://www.thetruthaboutcars.com/2012/09/suspension-truth-2-sport-suspensions-the-illusion-of-performance/
http://www.thetruthaboutcars.com/2012/09/suspension-truth-2-sport-suspensions-the-illusion-of-performance/#commentsFri, 07 Sep 2012 17:26:11 +0000http://www.thetruthaboutcars.com/?p=459039Edit: Now with updated graph So, what the heck does a manufacturer mean when they offer a ‘Sport Suspension’ and is it something you actually want? While I haven’t examined every version available, themes have carried through various makes/models, so what follows are safe generalizations. I even throw in a dyno chart! OEMs give us […]

So, what the heck does a manufacturer mean when they offer a ‘Sport Suspension’ and is it something you actually want? While I haven’t examined every version available, themes have carried through various makes/models, so what follows are safe generalizations. I even throw in a dyno chart!

OEMs give us lots of specs to get us warm and fuzzy about a car but the majority don’t affect your everyday, commuting-to-work driving experience. How they decide to set up a suspension does. They assume an average commute-only driver just wants a comfortable car. The enthusiast will opt for an (expected) upgrade via the sport version (with infinite colorful names). And if they happen to have a hard-core race version, that is another level all-together. What might feel fun on a 5 minute test drive (and help sell the sporty version) could get annoying (or literally painful) with ownership. Even more so with a disgruntled passenger (“I told you not to get this car!”). I believe (just as I’m writing this) that car makers know they have very little time to close a sale, like a first impression. If they can’t get your attention to begin with, they won’t capture it with pretty brochures or slick commercial spots. Your test drive experience is what will likely sell a particular package.

Fast forward to your first few months with this car; if you commute, most of the time you’re pointed straight and you really don’t want to be jostled all over the place. As I like to say (and have made a video using suspension potentiometer data to prove), there’s no such thing as a smooth road. The dampers and suspension are always working.

Even if most of one’s commute is a mountain road with 99 turns in 4 blissful miles, one still has to cross intersections, deal with potholes and other mundane events. Having a car that doesn’t beat you up is important, even if you’ve cultivated an immunity to the effects of poor damping. Why poison yourself to begin with?

In many sport suspensions, what you get is ‘the Illusion of Performance(IoP).’ I’d trademark it but would rather focus on the Perfect Ride! That IoP gives your body the sensation of activity – remember that we only sense acceleration, not velocity – but a damper with sharp edges on its force profile will cause time-varying load on the tires. The effect is being jerked around, a change in acceleration over time, like being on a rollercoaster.

An ideal suspension needs to soften the edges of the road, so the tires maintain contact and you get a human body-friendly ride (via muted vertical accelerations) plus solid lateral grip (minimal change in contact patch load during cornering). If the suspension designer felt its buyers would associate roughness with speed (which younger drivers – myself included – usually do), then it’ll emphasize creating jerk via more low-speed damping). For a more sophisticated audience or more expensive car, the low and mid speed will change more smoothly (still not necessarily optimal, esp. due to less compression than the chassis could use) they will typically add more high-speed rebound while keeping high-speed bump lower.

For this article I’ll make reference to 2 suspension options available on the 99-05 Mazda Miata, with a third introduced for 04-05 years. Standard was a twin-tube damper made by Showa, then a ‘Hard S’ package which used a Bilstein monotube and ostensibly stiffer and/or lower springs. From 04-05, the Mazdaspeed Miata came with its own package that had a 1mm larger front bar, 3mm larger rear, and stiffer/slightly shorter. The dampers had been tuned even more aggressively than the Hard S but were otherwise dimensionally identical.

The graph at the top of the page shows the various rear dampers only, but the fronts follow the same trend. A few observations: notice how the standard suspension has a much more smoothly varying shape, a more constant slope from 0 to +/- 2 in/sec (negative = bump, positive = rebound in this graph and all the ones we’ll share). The slope determines how much jerk the tires and you experience. The Hard S is 50% stronger in compression @ 1 in/sec and the MSM another ~15% on top. The difference in rebound and ratio of bump to rebound is what determines the degree of jacking down. At 1 in/sec, where small, repeated movements (like any rippled road surface) will affect the dynamic ride height, the ratio is a little less than 1:1 R:B for Showa, then 1.5:1 for Hard S and about 2:1 for MSM. The ‘sportier’ suspension specialize in more immediate steering feedback, yes (turning the wheel results in movements up to ~3 in/sec at the damper in the Miata’s case). But that degrades ride quality and road holding as well. Notice that the mid and high-speed damping isn’t very much different. In fact, the Showa has a strong slope for both bump and rebound, so it would tend to resist bottoming out better than the ‘sportier’ OE, Bilstein-based suspensions! One could also argue that the Bilsteins will blow-off better, which is true but I don’t find the amount of damping to be objectionable and in fact one could almost do a rally setup which was the inverse of the OE curves and have a wicked fast, comfortable car. Yes, I’ve done this! Yes, we’ve built this for customers. How stupid fast do you want to go?

I want to make it very clear that 99% of all complaints of poor ride have to do with jacking down via excess rebound damping, potentially combined with frequent engagement of the front bump stops which gets worse due to excess rebound/jacking down.

Jacking down can be an automakers best friend for numerous reasons. First, you get an additional ‘jerk’ when the bump stop acts as a supplemental spring. Two, the front end will get stiffer as the bump stop engages, increasing weight transfer across that axle and inducing more understeer. So even if you chuck the car into a turn (a novice driver won’t be trail-braking), it won’t want to turn. At least, not as eagerly as you’d like. Jacking down exerts a self-corrective effect on the driver. You are going slower in turns, you’re actually driving 50 though it feels you’re doing 80 (‘wow, what a sporty ride!’). And if you have bigger sway bars, then that jacking effect causes a coupled (cross-axle) time-varying load on both tires! Holy understeer, Batman!

I don’t fault them for doing this. Putting a very capable car in the hands of an inexperienced driver could be a bad thing. But they don’t tell you that this IoP is what they’re up to and that lack of fine print has bothered me since I learned these Truths. Pricier vehicles get better suspensions though it seems there’s always room to remove a bit of understeer, to have a bit more grip and sure-footedness, a bit more confidence. This isn’t including active suspension … although we did tune a Nissan GT-R last year using the OE Adaptronic Bilsteins. Results were very good and we could retain the Soft/Sport mode settings. It was a 2009 GT-R and the damping was definitely more biased to jacking the front down.

In the next article I’ll illustrate a few setups that have strong high-speed rebound and what effects you’d notice with that. This will include accelerometer traces showing the strong downward (negative) accelerations which are very hard on one’s body. I’ll also continue discussing the effects of bump stops on ride and handling.

HOMEWORK! For fun, check how close your front dampers (strut or shock) are to sitting on the bump stops. Report back in the comments section! I know for certain the Mini Cooper rests on the front bump stops, the Mitsubishi Evo and Subaru WRX essentially do the same.

Our newest segment, “Suspension Truth”, comes to us courtesy of Shaikh J Ahmad. An engineer by training, Shaikh is the owner of Fat Cat Motorsports, and a self-styled “Suspension Wizard”. Shaikh creates custom suspension components for a variety of cars, including the Mazda Miata and RX-8, the Nissan 350Z, Mini Cooper and Honda S2000. Back when I had my 1997 Miata, I ordered a set of coilovers from Shaikh, based on his reputation for creating suspension setups with a previously unheard of balance between ride and handling. The Fat Cat coilovers are one of the few products I’ve ever bought that were able to live up to the hype. Over the next few weeks, Shaikh will delve into the science of suspensions, and provide his own analysis of a number of production cars.

What’s your least memorable train ride? Simple question, right? If you’re reading this, I’m going to assume all of them. Unless a screenwriter threw you into an adventure film without your consent, it’s what we’d expect. This brings to mind a popular driving metaphor – ‘handles like it’s on rails.’ That’s our ideal in suspension tuning, to be glued to the ground and also as comfortable as possible. Easy when you’ve controlled every degree of freedom as with a train track and groomed earth beneath.

But what about your least memorable plane flight? Again, I’d hope most of them. How about the most memorable one – turbulence anyone? Whether chop, CAT, or simply bumpy air, turbulence can be annoying, as in delayed beverage service, or utterly terrifying. The unpredictable, jerky movements of an airplane caught in Mother Nature’s fury sharply draws your attention to the immediate environment. You aren’t relaxed anymore, thinking about the miles of air between you and the ground. You have to trust in your pilot, crew and the plane itself to handle the situation, working in harmony to return you safely back to Earth.

In between these extremes is the spectrum of what an automobile suspension can offer. As a driving enthusiast and amateur racer in my 20s, I only wanted suspensions that made my car handle better and go faster. Comfort was secondary and in fact I believed (as many do) that to be fast you must be uncomfortable. Ah, brainwashing by race companies and the follies of youth! Having trusted marketing hype from both automakers and aftermarket companies, I’ve come to see patterns in the past 15 years of my pursuit of Suspension Perfection. Ultimate speed and ultimate comfort. How are they linked, if at all? Can I make my trip to the race car unmemorably smooth and also have razor-sharp handling for a backroad jaunt, autocross run, track session or hill climb? What about safety, responsiveness and predictability?

Any automaker has to fulfill the task of keeping a vehicle on the road. They can do it in a bare-bones fashion, like a budget economy car that doesn’t inspire much confidence but gets you from point A to B. At the very high end, we have the Holy Grail: a buttery-smooth ride with incredible handling. Normally you pay superlative prices (Aston Martin, Ferrari, etc) for this achievement, but I’ve found that cost has very little to do with making an exceptional suspension. You need to understand the designer’s mandate, see if that matches your needs, then choose components (or a vehicle itself) that deliver. But we don’t get handled a personality test results for a Honda Civic, Toyota Camry or Porsche 911 Carrera. We have some bias based on past experience, what we’ve read, felt or been led to believe. But what really goes on in that murky black magic area of suspension design? By starting with an examination of the psychology behind a vehicle, why it exists, we can understand certain design choices then make targeted improvements to a production-based road car to the point it feels truly amazing.

Please note, this kind of suspension harmony matters whether one get groceries or chases championships. It’s been a fascinating process of discovering the truth of how grip produces both great ride and handling both. For a street-driven passenger car, how the suspension deals with the road, mile after mile, creates a somatic experience that can promote either ease or dis-ease. I’d rather see a driver smiling and relaxed after a trip than stressed and hurting. A relaxed (not numb), in-control driver is a safer driver and a happier human being. There’s also a very important somatic experience to the race car driver, who needs to have hyper-confidence in their machine’s responsiveness to dance it on the edge of adhesion.

One video in particular was very illuminating to me. It was of a journalist who had a chance to drive a few laps in a Formula 1 car. Once the lengthy process of preparing him for the experience was complete (simplified as it was in his not-very-physically-fit case), he took his laps, whooping the whole way through. Once he stopped the other reporters asked a seemingly rhetorical question ‘you just drove a Formula car! Wasn’t it really harsh?’ to which our lucky journalist gives a surprising answer: “No, in fact it was quite smooth once you were up to speed!”

Is it that really all that surprising to hear this truth? To give a driver confidence and ultimate speed, the proper suspension has to keep the tires in contact with the road. What’s good for the rather-soft tires (imagine driving around on a partially cooked egg) is good for the very soft driver. Going stiffer than is necessary robs grip and induces more discomfort. The just-stiff-enough setup will reward the aggressive, competitive or racing driver in many ways.

We’ll continue our explorations next time with a topic that is even more subtle – what does it mean to have a “Sport Suspension” and do you really want one?